1. Field of the Invention
The present invention relates to a Nuclear Quadrupole Resonance (NQR) method and apparatus for detecting a nitramine explosive. More specifically, the present invention relates to an NQR method and apparatus for detecting a nitramine explosive by detecting an NQR resonance signal produced by a nitro group of the explosive.
2. Description of the Related Art
There are many situations where it is desirable to detect the presence of a specific substance. For example, with the unfortunate increase in drug trafficking and terrorist use of high explosives in aircraft and buildings, there is a strong interest for a reliable detection system that can detect sub-kilogram quantities of narcotics and explosives against a background of more benign materials in a rapid, accurate, and non-invasive fashion.
Nuclear quadrupole resonance (NQR) is a known technique for detecting the presence of specific substances. More specifically, various substances produce an NQR resonance signal when excited by radio frequency (RF) radiation at a particular frequency. Generally, RF radiation at a particular frequency will cause a precession in nuclei of a specific substance, but not in other substances. Nuclear quadrupole resonance (NQR) takes advantage of this phenomenon to detect the various substances.
FIG. 1 is a diagram illustrating the overall hardware architecture of a conventional NQR apparatus. As illustrated in FIG. 1, a transmitter 20 and a receiver 22 are connected to a probe 24 through a transmit/receive (T/R) switch 26. To detect the presence of a specific substance, T/R switch 26 connects transmitter 20 to probe 24 while disconnecting receiver 22 from probe 24. Then, transmitter 20 generates a pulse train and supplies the pulse train to probe 24. Generally, the pulse train is formed by a signal having a frequency corresponding to the resonance frequency of the nuclei of the specific substance which is intended to be detected. Probe 24 receives the pulse train, and emits a corresponding signal towards a target specimen (not illustrated). If the specific substance desired to be detected is present in the target specimen, the signal emitted from probe 24 will cause the substance to produce an NQR resonance signal.
After the signal is emitted by probe 24, T/R switch 26 connects receiver 22 to probe 24 while disconnecting transmitter 20 from probe 24. Probe 24 then detects the NQR resonance signal produced by the specific substance, and probe 24 produces a corresponding output signal. The output signal of probe 24 is received and analyzed by receiver 22, to confirm the presence of the specific substance in the target specimen.
NQR detection has been used, for example, to detect nitramine explosives, such as RDX, HMX and tetryl.
Unfortunately, electronic items exposed to NQR detection, such as those in a suitcase or package undergoing inspection, may spuriously trigger. For example, a consumer electronic item such as a camera may initiate the flash or advance the film when exposed to NQR detection. It would be desirable to reduce the number of occurrences of such spurious triggering, especially in the detection of nitramine explosives.